In recent years, there arise problems of reducing CO2 gas, which is said to be a cause of global warming, or future exhaustion of oil or other fossil fuels. To address these problems, recyclable natural energy has been actively used. Wind power is a promising form of recyclable natural energy, and large-scale wind power plants have been increasingly constructed.
The most suitable area for wind power generation to be constructed is an area where strong winds are expected to blow constantly, and off-shore wind power generators are under development or actually in operation all over the world (see Patent Documents 1 to 4).
In order to build a tower for wind power generation at sea, it is necessary to drive the foundation portion of the tower into the sea bed. Further, in order to obtain a sufficient height for the turbine blades of the wind power generator from the sea level, the foundation portion of the tower is required to have a sufficient length.
Thus, the foundation portion of the tower of the wind power generator employs a steel pipe structure having a wall thickness exceeding 50 mm, for example, approximately 100 mm, and a large diameter in cross section of approximately 4 m. Further, the total height of the tower is as high as 80 m or more. In recent years, a large steel-structure such as the tower for the wind power generation is required to be built through electron beam welding on the coast near the construction site in an easy and efficient manner.
In other words, under the circumstances described above, there arises a new technical demand for welding an ultra-thick steel plate having a thickness of 100 mm on-site in a highly efficient manner.
In general, a high-energy-density beam welding such as electron beam welding and laser beam welding exhibits high efficiency. However, the thickness of the steel plate to be welded with the laser beam has been limited. Further, the conventional electron-beam welding is required to be performed in a vacuum chamber under a high vacuum state. Thus, conventionally, the thickness or size of the steel plate that can be welded through the high-energy-density beam welding largely depends on the capacity of welding equipment or the inner size of the vacuum chamber.
In recent years, to address the circumstances described above, an electron-beam welding method has been proposed that employs reduced pressure in the vicinity of a portion to be welded, thereby efficiently welding an ultra-thick steel plate with a thickness of approximately 100 mm on-site. For example, the Welding Institute of the United Kingdom has developed a welding method (reduced pressured electron beam welding: RPEBW) enabling operation under a low vacuum state (see Patent Document 5).
With the reduced pressure electron beam welding (RPEBW), it is possible to efficiently perform welding, by locally reducing the pressure of the portion to be welded to be in a vacuum state in the case where a large-scale steel structure such as the tower of a wind power generator is constructed. The RPEBW method is performed in a state in which the degree of vacuum is low as compared with the method of performing welding in the vacuum chamber.
In general, a fracture toughness value δc based on fracture mechanics is known as an index for quantitatively evaluating the safety of a welded structure. The δc can be obtained through a crack tip opening displacement (CTOD) test. The fracture toughness is affected by the size of a test piece. Thus, although favorable results can be obtained through a small-sized test such as the conventional V-notch Charpy impact test, it is not always true that the favorable fracture toughness value δc can be obtained through the CTOD test with a welded joint in the large-scale steel.
The electron-beam welding method is a method that employs energy of the electron beam to once melt and then solidify the base material of a weld target portion, thereby performing welding. Normally, the components of the molten metal in the electron-beam welding method are almost the same as those of the base metal (steel). On the other hand, with the large-heat input arc welding such as electro gas welding, mechanical properties such as hardness of the welded metal and the fracture toughness value δc are adjusted by using, for example, welding wire. It is difficult to use the welding wire in the electron-beam welding method.
In view of the above-described circumstances, a method of optimizing the hardness or cleanliness of the welded metal (WM) has been proposed to improve the fracture toughness value δc of the electron-beam welded joint (see, for example, Patent Documents 6 and 7). Patent Document 6 proposes setting the hardness of the welded metal to be more than 110% and not more than 220% of that of the base metal, and the width of the welded metal to be 20% or less of the thickness of the steel. Further, Patent Document 7 proposes setting the amount of O in the welded metal to 20 ppm or more, and the number of oxide having a grain diameter of 2.0 μm or more to 10 pieces/mm2 or less.